In molten iron produced by a smelting reduction method of iron bath type, a carbon level is more often low as 4 wt % or less, and in this case, a melting point is high. In general, the carbon level is 3 to 4 wt %. Hereinafter, the molten iron produced by the smelting reduction method of iron bath type will be called as “smelted and reduced molten iron”. Namely, the melting points of molten iron in general blast furnaces are assumed to be around 1150° C., and if the carbon content is different by 1 wt %, the melting point is different by around 70 to 100° C., and therefore carbon content as in the smelted and reduced molten iron is at a level of 3 wt %, and the melting point is around 1300° C. Metal or slag in the molten iron where the carbon level is low easily adhere troughs or ladles in treatments of molten iron, and such situations make the treatment of molten iron very difficult.
Sulfur level in the smelted and reduced molten iron is 0.3 to 0.4 wt %, which is around 10 times of sulfur in the furnace molten iron. Therefore in desulfurization outside furnace generally applied to the furnace molten iron, it is difficult to decrease the sulfur level below 0.01 wt % directing to steel making processes for producing ordinary thin sheets in view of treating efficiency and cost. In the direct iron ore smelted and reduced molten iron, Si is below 0.1 wt %, and often less than 0.01 wt %, and a heat allowance margin in a post-process is lower that that of the furnace molten iron.
In such conventional desulfurization treatments outside furnace, it is necessary to install facilities to be exclusively used for carrying out the desulfurization treatment on the way of feeding the molten iron to a converter yard, inevitably increasing the cost of equipment. Further, since a freeboard of a container must be made large for practicing the desulfurization treatment, a feeding container of the molten iron is accordingly large sized, and the costs for refractory to be used thereto are increased, and at the same time the cost of equipment is high. In addition, when carrying out the desulfurization treatment outside furnace, the temperature of the molten iron is lowered so that the heat allowance margin in a converter is decreased and scrap amount able to be consumed in the converter is dropped.
There have conventionally been developed and studied several techniques as process of producing molten irons not following the furnace process.
“Recent tendencies of new iron sources” by Iron Making Process Forum of The Iron and Steel Institute of Japan(Sep. 29, 1996), pages 42 to 51, and “Studies on iron making technique of directly using coal” by THE JAPAN IRON AND STEEL FEDERATION (June 1996) (called both as “Prior Art 1” hereafter) disclose alias “DIOS Process” (Direct Iron Ore Smelting reduction Process). The DIOS Process preheats fine iron ores to 700 to 800° C. in a fluidized bed prereduction furnace and reduces the fine iron ores until around 20% of the pre-heating reduction rate in a pre-reduction furnace, then charges the pre-reduced ores into the smelting reduction furnace, and blows oxygen gas into the smelting reduction furnace with coal as carbonaceous material and heat source. The smelting reduction furnace is of the iron bath type, and ordinary coal may be used as the carbonaceous material. This process may be continuously operated.
In this process, a fluidized bed furnace is used as the prereduction furnace, not taking a problem of permeability as seen in a shaft furnace but having a merit of using so-called sinter feed of ore grain size being below around 8 mm as an iron ore grain diameter. However, since an only waste gas from the smelting reduction furnace of a post combustion type is used as a reducing material, an accomplished pre-reduction in the fluidized bed furnace cannot exceeds 33% thermodynamically. In the fluidized bed furnace, a net unit of coal is 700 kg/t or higher, the net unit of oxygen is around 500 Nm3/t or more, and sensible heat and latent heat of generated gas are 1 to 2 Gcal/t. An oxygen plant or a gas treatment and energy conversion plant for establishing a process under such conditions are very large sized scales, and it is one of problems that taken is the high cost for incidental facilities around equivalent to the cost of facility of the smelting reduction furnace itself (Problem 1).
The DIOS Process is very superior in that the degree of freedom of raw materials and fuels is large, but since the cost of facility is tremendous as the Problem 1, it was difficult to actually utilize brown coal, sub-bituminous coal of low quality and limonite or iron ore hydroxide as ore of low quality.
Aiming at improvement of the above mentioned problems in the DIOS Process and, for example, decreasing the net unit of coal or the net unit of oxygen and heightening productivity, if increasing a post combustion ratio in the smelting reduction furnace and using the furnace wall of refractory as in the ordinary smelting furnace, the life of a furnace is from several tens to several hundreds hours. Therefore, a water cooling structure must be adopted for the furnace wall. If a water cooling panel is used to the furnace wall as a measure thereto, coal of high volatile content being 30 wt % or more is used, H2 and H2O of 20 vol % or more in total are contained in generated gas, and the post combustion ratio is heightened nearly up to 40% as in the Prior Art 2, heat load of the furnace wall is increased to 300 Mcal/m2h or more, and as heat loss from the furnace is increased and cooling water is much necessary for maintaining the water cooling structure, it has been assumed it unreal to more increase the post combustion ratio (Problem 2). If coal of low volatile content is used, an allowable post combustion ratio is 40 to 50%, and if it is at a level of coke, no problem occurs about the post combustion of 60 to 80%.
On the other hand, alias “ROMEL Process” is introduced at page 149 of “The 165th and 166th Nishiyama Commemorative Lecture” 1997 (called as “Prior Art 2” hereafter). In the ROMELT Process, the prereduction furnace is not installed, and as operations are carried out by the only smelting reduction furnace under an atmospheric pressure, it is unnecessary to dry coal and ores, and in these regards, this process is superior to the DIOS Process. However, since the net unit of coal is at least 1250 kg/t. the net unit of oxygen is at least 1000 Nm3/t and sensible heat and latent heat are large from several G to 10 Gcal/t, tremendous oxygen production and energy conversion facilities are required (Problem 1).
In the Prior Art 2, if the post combustion is increased, it could be expected to decrease the net unit of coal, but the heat load of the furnace wall is increased as the Prior Art 1 and the productivity is limited to around 1 t/h/m2 (cross sectional area of the furnace) (Problem 2).
Alias FASTME Process and INMETCO Process are introduced at page 117 of “The 165th and 166th Nishiyama Commemorative Lecture” 1997 (called as “Prior Art 3” hereafter). The Prior Art 3 is introduced as a DRI (Directly Reduced Iron) production technique of coal base, not natural gas, which pelletizes coal powder and fine iron ore, and reduces until generation of metallic iron in a rotary hearth furnace. Further, MPT International (1997), pp50-61, introduces a technique which directly produces DRI in the rotary hearth furnace, not pelletizing fine iron ore and coal (called as “Prior Art 4” hereafter).
However, in the smelting process of the Prior Arts 3 or 4, since S content contained around 0.5 to 0.6% in coal is partially removed, [S] concentration remains at least around 0.1% in products DRI. Therefore, when DRI is used as a molten raw material in a steel making furnace directly producing steel as an electric furnace, a big problem arises that a desulfurization treatment of the molten steel is very expensive (Problem 3). 100% of the using amount of iron source in the steel making furnace cannot be directly reduced, and the iron source is supplied only partially for steel production (Problem 4). CAMP-ISIJ (1997), 723, introduces a technology of pre-reducing ores with volatile content separated from coal (called as “Prior Art 5” hereafter). As this report carries out an analysis ignoring combustion heat burning C till CO, the balance of heat material is not conformed. The ore reduction ratio is not so high as around 40%. Due to this unconformity, a comparison when using coal and char in the smelting reduction furnace is not properly made.
The above mentioned problems are summarized as follows.
Problem 1 [with respect to the Prior Arts 1 and 2]: Since the sensible heat and latent heat are very large in the generated gas in the smelting process, an immense oxygen production facility and the energy conversion facility are necessary, and the cost of equipment is high expensive.
Problem 2 [with respect to the Prior Arts 1 and 2]: Since a small sized oxygen production facility and energy conversion facility are made enough, if the post combustion is heightened, the heat load in the wall of the smelting furnace is considerably increased and accordingly the heat loss from the furnace is increased. The cooling water is much required for maintaining the water cooling structure and the process is unreal.
Problem 3 [with respect to the Prior Arts 3 and 4]: When DRI (direct reduction iron) is used as a molten raw material, the cost for desulfurization is very high.
Problem 4 [with respect to the Prior Arts 3 and 4]: In the cost, the iron source is supplied only partially for steel production in DRI.
Problems to be solved in the smelting reduction process extend over wide ranges, and when the smelting reduction process is introduced in many steel works, namely, mini-mills other than so-called consistent makers of from molten iron to steel products having facilities from furnaces until rolling mills, such smelting reduction process of the low cost of equipment is required which decreases the consumption amount of oxygen necessary for smelting and reducing iron ores and the generation amount of gas therefrom, and economical in the cost of facilities. Further, seeing the smelting reduction process in view of environmental harmony, an amount of generating carbon dioxide gas is almost equal to that of the blast furnace process, or as the case may be, anxiety might arise that the generation amount thereof will be high. Even if it can be curtailed, the rate is around 5% at the utmost than the molten iron by the blast furnace. That is, as far as coal is used as a main reducing material or fuel in the smelting reduction process, it cannot be expected to largely curtail the generation amount of carbon dioxide.
The iron making facility having the blast furnace is installed with a sintering machine and a coke oven, and the sintered ore and coke produced respectively in the sintering machine and the coke oven are charged into the blast furnace for producing molten iron. In the smelting process by the blast furnace (the blast furnace process), sintered ores of small grain size cannot be employed for maintaining permeability within the furnace. Therefore, the sintered ores produced by the sintering machine are sieved, and those of small grain size are re-sintered as returned ores. The production yield of the sintered ore is generally limited to around 85%. Coke of small grain size cannot be used in the blast furnace for the same reason. The coke produced in the coke furnace is sieved, and powder coke of small grain size is used for producing sintered ores.
On the other hand, in the molten iron producing process using the smelting reduction furnace of iron bath type (the smelting reduction process or iron bath type), a pre-treatment as the blast furnace process is not necessary for ores or carbonaceous materials, and ores and coal can be used per se as raw materials. But since ores or coal are charged into the furnace where the operation is carried out under a closed condition, when powder ores or powder coal are used, a previously drying process and a facility therefor are required in order that ores or coal are not adhered in the charging route.